Electrodeposition of metal salts



Aug. 26, 1952 KAHN 2,608,530

ELECTRODEPOSITION 0F METAL SALTS Filed March 24, 1945 y/ jiizki' llihllllllll I-UI INVENTOR. MILTON KAHN WWI 4 W ATTORNEY.

Patented Aug. 26, 1952 ELECTRODEPOSITION OF METAL SALTS Milton Kahn, Berkeley, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application March 24, 1945, Serial No. 584,685

This invention relates to the electrolytic deposition of metal salts from aqueous solutions, and more particularly to the electrolytic deposition of metal salts from aqueous solutions of ionizing salts thereof in which one of the ions exists in at least two stages of oxidation, in one of which stages the salt is soluble and the other results in a deposit of an insoluble salt of the metal. The invention is especially directed to a process of preparing uranium in a form for isotope composition analysis. More specifically, the invention appertains to methods and means of preparing compounds of uranium and other metals for use in various other instruments and procedures;

It is often necessary to prepare layers of metal or metal compounds for work in which only thin, uniform films can be employed. One such procedure is for the isotopic analysis of metal, such as uranium, as described in the copending United States application Serial Number 536,440 filed'May 19, 1944, by Segre and Kennedy (now abandoned). In that method of analyzing polyisotopic uranium, independent measures of mass of sample, of radioactive disintegration or alpha activity of sample, and of slow-neutron produced fission of the sample are required. The last two of these measurements may be made utilizing a target comprising a thin layer of uranousuranic oxide (U308) on a platinum foil backing. In preparing these targets of uranium for isotopic analysis, it is necessary to prepare the uranium sample as a suitable composition (preferably as substantially pure uranous-uranic oxide) in the form of a film of such thickness that very little alpha particle absorption occurs within the film. A film thickness corresponding to approximately four hundred micrograms of uranium per square centimeter is taken to be about the maximum allowable.

In the prior art process of preparing these target films, chemical methods have most generally been employed but considerable difficulty has been encountered in obtaining uniform, thin layers of the desired thickness thereby.

It is an object of this invention to provide methods for improving the Kennedy and Segre method of isotopic analysis, by simplifying the preparation of suitable metal compound films and by eliminating the attendant failures in the prior art methods of film preparation.

Still another object of the invention is to provide a process of electrodepositing a predetermined amount of a sample of uranium in the form of a film, whereby the isotopic composition of the uranium may be determined.

1 Claim. (01. 2o4--5s) A further object of this invention is to provide a process for electrodepositing a predetermined amount of a sample of uranium as a film of insoluble uranium fluoride and igniting the uranium fluoride to uranous-uranic oxide, whereby the isotopic composition of the uranium may be determined;

It is an additional object of this invention to provide a process for electrodepositing insoluble metal salts from a solution of a soluble ionizing salt of that metal, one of the ions of said salt existing in at least another stage of oxidation, and said other stage of oxidation resulting in a deposit of an insoluble metal salt.

Other objects and advantages of the invention will appear from the following detailed de scription including claim and drawings.

In general, the present process is directed to the electrolysis of an aqueous solution. of a soluble ionizing salt of a metal, which metal is not readily electrodeposited, at least one of the ions of said salt existing in two stages of oxidation, in one stage of which an insoluble metal compound is deposited, the unaltered ion being in large excess at the electrode. Incidentally, it is to be understood that when speaking of the two stages of oxidation of the ion, it is naturally intended to exclude the free metal (or zero stage) as one stage of oxidation. The process includes electrodeposition by reason of reduction of an ion either negative or positive at the cathode and/or oxidation of an ion either negative or positive at the anode. The result is that an insoluble metal compound is deposited as a uniform layer, either at the cathode or at the anode. Hence, it may be stated by way of general illustration that the electrolysis process can be applied to aqueous solutions of salts of any metal (or metals) not readily electrodeposited, which metal meets one or more of the following requirements:

a. A metal which has two oxidation stages and which forms a salts with a negative ion of an acid with only one of the oxidation stages forming an insoluble salt for the concentration involved, or

b. A metal (that preferably has only one oxida tion stage) which forms a salt with a negative ion that has two stages of oxidation with only one of the stages resulting in the formation of an insoluble salt of the metal in the concentration involved, or

c. A'metal which forms a complex ion, organic or inorganic, having two states of oxidation, only one of which results in an insoluble salt of that complex or decomposes into an insoluble salt electrode for the deposition of heavy films, e. g.,

in excess of 250 micrograms-per square centimeter.

Among the processes which can be employed the followingare examples:

An aqueous uranyl fluoride solution containing an excess of fluoride ions may be'electrolyzed An insoluble lower;

using a platinum cathode. uranium fluoride will be deposited as a film on the cathode. This film may be ignited to a deposit of uranous-uranic oxide.

A solution containing thorium tetranitrate and a relatively small amount of potassium ferricyanide is electrolyzed so that a film of complex thorium and iron cyanide (thorium ferrocyanide) is deposited on a platinum cathode. This may be ignited to a mixture of iron and thorium oxide from which the iron oxide may be leached out to leave a thin thorium dioxide film on the platinum disk. Incidentally, this is illustrative of the second and third requirements aforementioned, depending on whether one considers the thorium or the iron.

The foregoing examples are given for purposes of illustration but are not limiting on the scope of the invention. r

Additional 'features and advantages of the present invention will appear from the following detailed description taken in conjunction with the drawing forming part of this specification in which drawing Figure 1 is a side elevation of the electrodeposition apparatus, Figure 2 is a horizontal sectional view taken onthe line 2-2 of Figure 1 showing details of the electrodeposition cell, Figure 3is a vertical sectional. view of the electrodepcsiti-on cell [0 taken on the line 3-3 of Figure 2 and Figurei show a conventionalized perspective view with parts out of scale andproportion for clarity of the finished target.

In the drawing adisk l l which is to be coated with uranium comprises an approximately 0.002 inch platinum foil cut to a diameter of about one and one-half inches. The disk is thoroughly cleaned, ignited to red heat and tared. The disk is then ready to be assembled as part of the electrod-eposition cel-l I 0 in-Which the disk is to serve as the cathode upon which uranium is to be deposited. 1 v I The above-mentioned platinum disk H is placed on a larger and thicker platinum disk l2, which is about 0.010 inch in thickness and about two inches in diameter and which is cut so that a tab l3zextends out from the disk and makes connection with the wire [4 to which a negative potential may be applied. In turn, the-platinum disk 12 is placed on the ground glass plate [5 which serves as a supporting base for the electrodeposition cell [0. The glass cylinder I6, which is provided with a suitable number (preferably four) of append-ant glass hooks l1 and-has an outside diameter which is a little less than one and one-half inches, is concentric-ally placed upon the platinum disk. -l leaving a surface about one and one-quarter inches in diameter, i. e., about eight square ,centimetersin area, of

disk exposed within cylinder l6, and is secured to this position by passing a band 22 of rubber or the like alternating under the hooks l8 which are arranged in the glass plate l5, as indicated on the drawings, and over the hooks I! of the glass cylinder IS. A liquid-tight seal is formed at the-junction of glass cylinder 16 and platinum disk II by forming a filletv I! of beeswax or the like around the outside of the junction. This is accomplished by applying, with a small brush, molten beeswax to the junction, the beeswax being well above its melting point in order to assure a good seal.

The anode 20 compriseszan approximately nine inch length of platinum wire of one-sixteenth inch diameter of which about six and one-half inches of the length are wound in a spiral leaving about two and one-half inches of straight wire for connection to a stirrer. The platinum anode is fastened in the chuck of the small variable speed electric stirrer 2| and the position of this stirrer is adjusted so that the spiral anode dips about half way down into the electrodeposition cell I0. The anode is electrically connected so that it is at a potential positive to that of conductor [-4 and is ordinarily grounded. The electrodeposition cell I 0 is now completely assembled and is ready for use.

In the case where uranium is to be deposited as uranium tetra-fluoride on the platinum disk I I, about ten milliliters of 0.03 M sodium fluoride solution, i. e., about 300 micromoles of fluoride ion, and about five milliliters of substantially pure uranyl nitrate solution which has beenprepare-d from the uranium sample to be analyzed for isotopic composition are placed in the cell. The concentration of the uranyl nitrate solution depends on the amount of uranium which is to be formed as a film on the platinum disk ll. Ordinarily, five milliliters of this solution is made up to contain about two hundred and fifty micrograms, i. e., about 1.05- micromoles, of uranium which, when deposited, produces a film of a density of about thirty-one micrograms of uranium per square centimeter. The solution, however,- may contain as much as about thirty-two himdred micrograms, i. e., about 13.4 micromoles, of uranium. In this case, a film of a density of approximately four hundred micrograms of uranium per square centimeter is formed'when the uranium is deposited. As previously mentioned, such a film is of about themaximum allowable thickness. The ratio of fluoride ion concentration to uranium ion concentration is thus seen to lie in the range of from about 300 to about 25. A potential of about seven volts is applied to the electrodeposition cell It), whereby a current of about fifty milliamperes flows through the cell (which is approximately equivalent to a cathode current density of about 0.6 ampereper square deeimeter). At the same time the variable speed electric stirrer 2| is turned on, whereby the platinum wire anode .20 is caused to rotate at a suitable speed. The action of the applied potential is to cause the uranium as uranyl ion, UOz++, to migrate to the platinum disk H and be reduced to U++++. or U+++ ion, which combines with neighboring fiuo ride ions and precipitates as uranium tetraor tri-fluoride on the platinum disk I I. 7 Current is allowed to flow through the electrodeposition cell 10 until the uranium in the cell-is substantially completely deposited on the platinum disk I I. This is accomplished in about ninety minutes. After the current is turned off the solution is poured out of the cell and the cell is rinsed with waten-The platinum disk II is then detached from the cell and again washed with water. The disk is next ignited in an oxidizing flame for about one minute, whereby any beeswaxon the disk is burned off and uranium tetrafluoride deposit 23 is quantitatively convertedto uranous-uranic oxide, U308.

The-platinum disk H, accordingly, is covered with a thin uniform film-of U508 and is ready for the isotopic analysis procedure which includes weighing the UsOa, determining thealpha activity of the U308 and determining the; fission activity of the-U308 under slow neutron b=ombardment. The foregoing procedure for .elec-trodeposition of uranium as uranium on "and/ojr'tetra-fluoride is considered to be 'verysatisiaotoryl but it may be modified by substituting for the fiuoride ion :other negative ions which fOlIllliIlSQlllblE-i salts of the uranium in areduced state 1 but not in the uranyl state and which ions areless susceptible .to changein the electrolysis than the uranium ion. I These ions should be. in large excess over the other uranium ions at the electrode in order to have the process efiective.

The filmproduced from the fluoride process has been found to. be verysatisiactory for alphacounting and the deposition of uranium is quantitative. The uranium tetrafluoride deposited is exceptionally pure and is substantially free of substances which would result in an impure U303 after ignition. The method is extremely easy to carry out and requires a relatively short time.

The platinum disk supporting the thin uniform film of U308 constitutes a target which is then utilized for the purpose of determining the isotopic composition of the uranium in accordance with the previously mentioned Segre-Kennedy application. This method includes weighing the target, then supporting the target upon a copper base plate and subjecting it to alpha activity and fission activity measurements, whereby both the alpha activity of the U308 and the fission activity under slow neutron bombardment of the U308 are determined.

In view of the foregoing, it is apparent that there has been provided an improved method for preparing uranium in a pure form as a thin film of U308 in which form the uranium may be analyzed for its isotopic composition. The preparation is accomplished by electrodepositing the uranium either as uranium tet-raflu-oride or other salt on a backing member, such as a thin platinum disk, and subsequently igniting the uranium salt to U305.

The corresponding process can be applied to other metals which form soluble higher valence .oxyfluorides and insoluble lower valence fluorides by substituting a corresponding molecular quantity of these elements for the uranium.

Another example of the present invention is for the electrodeposition of a salt by reduction of the anion, the metal ion being in excess. In the procedure four cc. of potassium ferricyanide solution (containing 0.1 gram of IQFe (ON) 6 in 200 milliliters of water) and sixteen milliliters of thorium nitrate solution (containing five grams of Th(NO3) 4.4:H2O in 100 milliliters of water) are added to an electrolytic cell as shown in the drawing. The source of current is a six volt storage battery. A platinum spiral 20 is the anode, and the platinum plate 23 is the cathode.

The platinum anode is fastened-in the -chuck rotated at fairly low speed.

The solution is electrolyzed at three andorie half volts and fifty milliamperes for about onehalf hour, the solutionbeing"stirred duringthe electrolysis. The cathode" plate removed and after washing it well with water is' ignited over anoxidizing flame such as -iro'm a' bunsen burner; The plate may then be placed in a-dish of twelve normal hydrochloric acid and heated over a steam bath for one-half hour to remove the-iron oxide. This pro-cessshould be repeated using twelve normal hydrochloric acid and a heating time of fifteen minutes. Practically all of the iron should be dissolved off the plate after this treatment. Some thorium oxide will be removed by this process of leaching. j If a' thickenfilm than' the one obtained'is desired, the plate-is remounted in the cell and the entire procedure repeated; 'This repetition of procedure canbe used to build up a thick plate. By varying the electrolysis time from zero to one-half hour-very thin films can beobtained. Films ranging from about twenty micrograms per "square centimeter to five hundred micrograms per 's'quare cen-ti meter of thorium oxide havebeen prepared by this procedure; f

This method of electrolytically preparing films of uranium, and thorium from aqueous solutions isnot restricted to'these elements alone. A film may be prepared of any element that has two oxidation states excluding the zero state and forms an insoluble salt in one of them, the insoluble salt being stable at the anode if formed at the anode or stable at the cathode if formed at the cathode. If the element has only one oxidation state excluding the zero state, then use may be made of anions with two oxidation states (e. g. sulphate-sulphite, ferricyanide-ferrocyanide, arsenite-arsenate) one of which forms a precipitate with the element in question. The precipitates formed must be stable toward further oxidation or reduction depending at which electrode it is formed.

The precipitate of uranium fluoride is an example of the firs-t possibility mentioned and the thorium ferrocyanide i an example of the second where anion is reduced and precipitates the cation. The important factor in this type 01. electrolysis is to keep the ratio of the concentration of the reaction material (in the case of uranium fluoride precipate, the uranyl U02 ion is reaction material) to tying-up material (F- ion in this case) very small so that when the changed ion (U* or U ion) is produced its rate of diffusion from the electrode (in this case cathode) will be smaller than the rate of diffusion of the tying-up ion from the solution to the electrode (the F- ions to the cathode). The pH of the solution should be maintained at a value that the hydroxyl ions are not present in a quantity suificient to cause precipitation of the hydroxide (or hydrated oxide) instead of the salt.

This electrolytic process applied to aqueous salt solutions for preparing films is to be distinguished from the preparation of films of metal compounds by electrophoresis of colloidal suspensions of metal compound in water and from. the preparation of films by electrolysis of fused salt baths.

While there has been described what is at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made therein. Thus, for

example, backing'members of noble metals other than platinum, such as gold, palladium or the like, may be used. Also, for example, soluble flu: orides' otherqthan sodium fluoride, as given in the first examplamay be'used. H Y IIhejterms-uranium, thorium orthe like as; used in the jspecificationand claim are used generically to refer to the metal whether inclemental or combined state except as indicated otherwise by the context. I c As many widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the applicant does not limit himself to the specific 1 disclosures and embodiments except as defined in the appended claim. -Ic1aim: v I The process for quantitatively electrodepositing uranium fluoride from a dilute aqueous solution of; ur-anyl nitrate which comprises contacting at least two electrodes with a dilute aqueous solution of uranyl nitrate wherein concentration of sodiumvfluoride lies in the range of between about 25; times to about 300times the concentration of the uranyl nitrate and passing a direct current through said solution at a potential of about seven volts and a current density at the cathode of about 0.6 ampere per square decimetenwhereby insoluble lower valent uraniumfluoride is deposited..-

MILTONKAHN.

byPierle (1919), pp. 527-536.

' REFERENCES CITED f The following references are of record in-"the file of this patent: 1 ,3

UNITED STATES PA'I'ENTS' V Number Name 7 Date, 571,531 Langhans Nov. 17, 1896 698,696 Franchot Apr.v 29,1902 1,050,796 I Bleeker Jan. 21, 1913 1,896,022 Theisz Jan. 31, 1933 1,912,430 [Cain June 6, 1933 2,059,053 Stareck Oct. 27, 1936 2,081,121 Stareck May 2-1, 1937 2,374,289 Hull Apr. 24,1945

OTHER REFERENCES .Electrolytic Oxidation and Reductionfffby Gl-asstone et al. (1936), pages 135, 137, '108,f 

